1. Field of the Invention
The present invention relates to biocompatible materials for medical implants, and particularly to tissue ingrowth-enhancing biocompatible materials.
2. Description of the Prior Art
The most commonly used rigid biocompatible implant materials are ceramic materials that may be used as bulk materials as for instance Al2O3. They also may be materials normally found as or used as layers on substrates, such as TiO2, TiN (materials that are considered to be ceramic in nature) or hydroxylapatite. These materials normally are not piezoelectric and normally cannot be polarized so as to become piezoelectric.
Ceramics having mechanical properties comparable to the above ceramic materials and having good piezoelectric properties, i. e. mainly polarizable ceramic ferroelectrics, normally contain components that can be suspected to be potentially harmful in the implanted state, for instance lead, as for instance in lead titanate, lead metaniobate or lead zirconate. It thus is undesirable that these materials directly or indirectly come into contact with body tissue or fluids. This is particularly important for piezo materials that are to be left in the body for a very long time.
Biocompatible phosphate glass ceramics that may contain crystal phases of apatite and AlPO4 in the tridymite and/or berlinite form are disclosed in U.S. Pat. No. 4,698,318. Berlinite is an isotype to quartz and has inherent piezoelectric properties. It is suggested that the piezoelectric properties of the berlinite can be utilized to promote healing of bone fractures. Berlinite has relatively weak piezoelectric properties. Since berlinite only is a part of the material, the overall piezoelectric properties of this material are weak. The piezoelectric properties are obtained by thermal treatments at relatively high temperatures for long time periods said to cause targeted precipitation of apatite or of apatite and AlPO4-crystals. The long-term stability of the material in the implanted state is not discussed, but hydroxylapatite and apatite are at least to some extent biodegradable.
There are some other biocompatible materials with piezoelectric properties. These materials mostly are relatively soft piezoelectric polymers with inherent piezoelectric properties. Other materials are biocompatible polymers containing inorganic polarizable, ceramic ferroelectric particles such as lead zirconate titanate (PZT) or barium titanate. Such polymeric materials are for instance disclosed in U.S. Pat. No. 5,684,061 where they are used as membranes for enhancing bone regeneration.
These materials have relatively weak piezoelectric properties and have a limited use in view of their relative softness, their potential toxicity and their poor mechanical and chemical strength.
U.S. Pat. No. 2,976,246 describes the production of potassium-sodium niobate ceramics exhibiting piezoelectric properties for use in the manufacture of electromechanical transducers for delay lines. Specific compositions are in the range K0.9Na0.1 (NbO3) to K0.1Na0.9 (NbO3), the formula of the preferred embodiment being K0.1Na0.9 (NbO3).
An object of the invention is to provide a highly biocompatible material that has a long-term stability in the implanted state and that can be wholly or selectively polarized in order to obtain piezoelectric properties for tissue growth promoting purposes.
It is a further object of the present invention to provide a biocompatible ceramic material for an implant which allows the degree of fixation of the implant tissue to be determined.
Another object of the present invention is to provide a medical implant employing biocompatible ceramic material of the type described above.
These objects are inventively achieved in a biocompatible ceramic material and in a medical implant containing such biocompatible ceramic material, wherein the ceramic material comprises NaxKyNbO3, wherein 0xe2x89xa6xxe2x89xa60.8, 0.2xe2x89xa6yxe2x89xa61, and x+y=1.
In the implant employing such ceramic material, the ceramic material is provided as a film or a layer on an implant body, or on a part comprised of a material other than the ceramic material. The ceramic material can be selectively polarized so as to provide the implant with spatially varying piezoelectric properties, meaning that the implant at one portion or location of its overall structure has a piezoelectric property or characteristic which differs from a piezoelectric property or characteristic at another location or part of the overall implant structure.
By means of these spatially varying piezoelectric properties, an indication of the magnitude of the forces that are transferred from the implant to the adjoining tissue, when the implant is implanted, can be measured, and this measurement is an indication of the degree of fixation of the implant to the surrounding tissue.